Oxygen generation for aircraft breathing applications requires that the product gas concentration stays within predetermined altitude-dependent minimum and maximum physiological limits. Normally, the minimum oxygen content of the breathable gas is that required to provide, at all cabin altitudes, the same or greater oxygen partial pressure as at sea level. A maximum oxygen concentration is set to reduce the likelihood of partial lung collapse during low-altitude high G maneuvers. More particularly, the risk of partial lung collapse increases with the risk of total adsorption of entrapped pockets of gas in the lungs, which result from distortion of the lungs during high G maneuvers. The risk of total adsorption of the entrapped gas increases with increased oxygen concentration (i.e. reduced nitrogen concentration).
Methods are known for the generation of low-pressure oxygen-enriched air. One such method is referred to as pressure swing adsorption (PSA) and has the advantage of being able to provide oxygen-enriched air in a short period of time after the supply of a suitable feed gas (e.g. pressurized air). The pressure swing adsorption process uses pressure to control adsorption and desorption. According to this process, the nitrogen in pressurized air is adsorbed in a molecular sieve bed while the oxygen passes through the bed. When the molecular sieve in the bed has become nearly saturated, the bed is vented to atmospheric pressure. This causes most of the nitrogen-adsorbed gases to be desorbed and discharged from the bed. In a two-bed system, when one bed is producing oxygen, some of the enriched product gas is flushed back through the (vented) other molecular sieve bed to further lower the partial pressure of the adsorbed gases in the vented bed and to complete the desorption process. Using two beds which are pressurized and flushed alternately provides a continuous flow of product gas and ensures sufficient pressure for the flushing operation.
Aircraft on-board oxygen generation systems (OBOGS) are known in the art. These systems are based on the molecular sieve gas separation process discussed above. Such systems are said to be "self-regulating" since the pressure swing increases with altitude, and therefore the efficiency of the process also increases to ensure sufficient oxygen concentration at high altitudes. More particularly, since each sieve bed is vented to the atmosphere (or cabin) during its regeneration phase, the bed pressure during desorption decreases with increasing altitude, thereby enhancing the desorption process.
In order to keep the oxygen concentration within maximum limits at low altitudes, processes have been developed to reduce the performance of prior art OBOGS. U.S. Pat. Nos. 4,661,124 and 5,004,485 (Humphrey, et al), disclose an alternating bed oxygen generating system with controlled sequential operation of charge and vent valves according to a series of selectable overall cycle times ranging between a minimum and a maximum, in a number of discrete steps. By extending the overall cycle time, efficiency of the system is reduced thereby regulating the product gas oxygen gas to within physiological maximum limits.
In U.S. Pat. No. 4,661,124, the overall cycle time of the molecular sieve beds is controlled using a pressure transducer on the basis of cabin pressure which is indicative of the altitude at which the aircraft is operating.
In U.S. Pat. No. 5,004,485, an oxygen sensor is used to test the gas concentration and a comparator function is implemented to compare the sensed oxygen concentration with values in a look-up table of desired product gas oxygen concentrations at various altitudes. In response to implementing the comparator function the overall cycle time is controlled to provide suitable concentration levels.
Prior art systems employing overall cycle time control, such as disclosed in U.S. Pat. No. 4,661,124 and 5,009,485 (Humphrey et al) suffer from a disadvantage in that it is difficult to accurately control the output oxygen concentration because performance changes occur over a small range (e.g. 4.5 seconds to 5.5 seconds in some systems, whereas cycles ranging from 5.5 seconds to 8.5 seconds do not result in any performance changes).
Dynamic control of system performance to regulate product gas to within the minimum and maximum physiological limits, requires reliable performance of the oxygen sensor connected to the concentrator output. The use of Built-In-Test (BIT) switches is known in the art for initiating integrity tests for current-limiting oxygen sensors. U.S. Pat. No. 5,071,453 (Hradek, et al) discloses a Built-In-Test function for implementing a system self-test for preflight and an oxygen sensor calibration check for operational level maintenance. When the BIT switch is momentarily depressed, a solenoid valve is energized to direct a flow of air through the oxygen sensor. Once the air reaches the sensor, the sensor output begins to drop indicating reduced oxygen concentration. Upon dropping to below a predetermined warning level, an alarm is activated indicating that the self-test has been successful. If the sensor output does not fall below the predetermined warning level within a defined time period such as 20 seconds, the self-test is deemed to have failed and the warning remains activated. The oxygen sensor calibration check involves energizing the same solenoid valve for passing a flow of air through the sensor in response to a lengthened depression of the BIT switch. During the calibration check, the air flow through the sensor is maintained for a longer period (3 minutes) than during the self-test, so that the sensor output falls below the warning level to within a predetermined threshold calibration range, causing activation of the warning signal. If the output of the oxygen sensor does not fall to within the predetermined calibration range within three minutes, the calibration test is deemed to have failed and the warning remains activated after completion of the test.
Although systems such as discussed above are known for testing the operation of oxygen sensors in OBOGS, the lack of maintenance testing systems for oxygen concentrators results in high support equipment requirements including the use of inlet air sources and test sets (to set the concentrator product flow and measure oxygen composition). Inlet air sources can often become contaminated with moisture and oil, and test sets require calibration control.